Two of the most common complaints of individuals with hearing impairment or cochlear implants are that they are unable to hear out voices in the background of other sounds, and are unable to enjoy music. In most cases these listening difficulties, which lead to significant communication challenges in everyday life, are not remedied satisfactorily by current auditory prostheses. Part of the reason for this situation is tht our current understanding of the origin of these perceptual difficulties is not sufficiently advanced to guide the development of sound-processing algorithms that can effectively alleviate them. The long-term goal of this project is to improve our understanding of the mechanisms underlying the perception of pitch of single and concurrent sounds in the normal and impaired auditory system. Pitch is a primary auditory percept that plays a crucial role in the perception of music and speech. Differences in pitch are currently thought to provide an important cue for the perceptual separation and tracking of concurrent voices. Individuals with cochlear hearing loss generally have a reduced ability to discriminate the pitch of complex sounds, especially in situations where more than one sound is present. This study will test the hypothesis that this perceptual deficit is related to poorer cochlear frequency selectivity resulting from cochlear damage. In addition, we will test current hypotheses concerning the relative importance of frequency selectivity and spectral (also known as place or tonotopic) mechanisms, versus temporal-coding mechanisms for pitch and speech perception in both normal and impaired hearing. The proposal is divided into three specific aims. The first aim combines behavioral (psychoacoustic) measures, computational studies using physiologically realistic models of peripheral auditory processes, mathematical optimal-observer models of central auditory processes, and advanced Bayesian data-analysis techniques, to provide rigorous tests of current pitch-perception theories and new insights into underlying mechanisms of auditory perception in normal-hearing and hearing-impaired listeners. The second aim investigates the ability of normal-hearing, hearing-impaired, and cochlear-implant listeners to discriminate the pitch of complex sounds presented concurrently with a competing sound, thus simulating under well- controlled laboratory conditions everyday-life situations in which hearing-impaired listeners often experience considerable difficulties. The third aim further addresses the relative importance of spectral and temporal (including fine-structure and envelope) information for speech perception under realistic listening conditions, including in quiet and in the presence of a competing voice, with and without reverberation, and with frequency shaping to simulate hearing-aid processing. The results will have important implications for our understanding of pitch processing, and should also be useful in the development and assessment of novel cochlear-implant processing schemes that attempt to enhance the perceptual segregation of concurrent sounds.